CHAPTER 7 The Somatosensory System •Fig. 7.2 Cutaneousmechanoreceptorsandtheresponsepatternsofassociatedafferentfibers.A,Schematicviewsofglabrous(hairless)andhairyskinshowingthearrangementofthevariousmajormechanoreceptors.B,Firingpatternsofthedifferentcutaneouslow-thresholdmechanosensitiveafferentfibersthatinnervatethevariousencapsulatedreceptorsoftheskin.(TracesinBarebasedondatafromJohanssonRS,VallboÅB.Trends Neurosci 1983;6:27.) •Fig. 7.3 Receptivefieldcharacteristicsfortype1andtype2sensoryafferents.Plotsinthetoprowshowthethresholdlevelofforceneededtoevokearesponseasafunctionofthedistanceacrossthereceptivefield.Receptivefieldsizeisshownonthehandbeloweachplot.(DatafromJohanssonRS,VallboÅB.Trends Neurosci 1983;6:27.) characteristics help explain the firing properties of these sensory afferents. FA1 afferents terminate in Meissner’s corpuscles, whereas SA1 afferents terminate in Merkel’s disks. In both cases the receptor is located relatively superficially, either in the basal epidermis (Merkel) or just below the epidermis (Meissner) (see
Fig. 7.2 ). These receptors are small and oriented to detect stimuli pressing down on the skin surface just above them, thus allowing SA1 and FA1 afferents to have small receptive fields. For glabrous skin, SA2 afferents terminate in Ruffini endings and FA2 afferents end in Pacinian corpuscles. Both of these receptors lie deeper in the dermis and connective tissue and therefore are sensitive to stimuli applied over much larger territory. The capsules of both Pacinian and Meissner receptors act to filter out slowly changing or steady stimuli, thus making these afferents selectively sensitive to changing stimuli.
For hairy skin, the relationship between receptors and afferent classes is similar to that of glabrous skin. SA1 and SA2 fibers connect to Merkel and Ruffini endings, the same as for glabrous skin. Pacinian corpuscles also underlie the properties of FA2 afferents; however, they are not found in hairy skin but instead are located in deep tissues surrounding muscles and blood vessels. There is not an exact analogue to the FA1 afferents. Rather there are hair units, which are afferents whose free endings wrap around hair follicles (see
Fig. 7.2 ). Each such hair unit will connect with about 20 hairs to produce a large ovoid or irregularly shaped receptive field. These units are extremely sensitive to movement of even a single hair. There are also field units that respond to touch of the skin, but unlike FA1 units, they have large receptive fields.
Several psychophysical and neural coding questions can be related to the receptive field properties and sensitivities of the various categories of afferents. For example, is the threshold of perception of tactile stimuli due to the sensitivity of the peripheral receptors or to central processes? In fact, by using microneurography it is possible to show that a single spike in an FA1 afferent from the finger can be perceived, thus indicating that the receptors limit the sensitivity; however, for other skin regions, perception is more dependent on central factors such as attention.